While trimming a cavalier is frowned on in the show ring, many pet owners decide they'd like to have something easier to care for for a range of reasons. This link from the ACKCSC (national US club) has suggestions for trimming a cavalier; I've pasted the relevant bit of the article below as well.

http://www.ackcsc.org/articles/Not%20That%20You%20Would%20EVER.pdf

Not That You Would EVER Trim A Cavalier....

... The following is a trim that we've come up with that works well, especially in winter, when it can get so messy. We suggest it to many pet owners -- they simply take the recipe to their groomer. Its really nothing more than a modified cocker trim; if you know how to use clippers, it's easy, and if you don't, its not hard to learn. The best results come, when the dog is brushed out, the rough trim put on including the scissor work, the dog bathed and fluff dried, and then completely re-trimmed.

With a # 10 blade, clip between the pads of the feet and the insides of the hind legs, going as far forward to the chest as you want, and depending on the amount of coat.

With a # 7 blade clip the top of the head, sides of the face and under chin to the top of the bib. The ears my be left untouched, or can be clipped down about one quarter. If they are clipped down at the top, it eliminates the matting which sometimes occurs behind the ears, but the head looks prettier if the top of the ears are left alone. The ears may be shortened if desired. Clip the neck on all sides except the front. Clip the back down to the top of the tail going down both sides as far as you like, and suits the dog, but usually to the top of the widest spring of rib.

Continue this line to the hind legs as well and in the front, to a point midway between the shoulder and top of leg. The rest is scissor work and depends on the amount of coat and personal preference. You may leave as much or little as you like. Scissor the bib back and the feathers as well. The undercarriage should be shaped to suit the dog and the tail shortened. You can get as fancy as you like now, using thinning shears to blend, etc. The feet are rounded, taking the slippers back as much as you like. For even shorter feet, push the hair up between the toes and scissor it level with the tops of the toes. Craigowl Prudence of Ravenrush, who turned 16 in August, has worn
this year round for years now, and is trimmed about 3 or 4 times a year -- each time turning back into a puppy when finished! You'll be amazed how great they feel and love showing off their 'new do.'